In previous versions, print was a statement. No parentheses were required and you couldn't pass print around as a first class object. That changes with Python 3. The following snippet is valid Python 2 code:

Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> f = open('outfile', 'w')
>>> print >>f, "The Output"
>>>

Running the preceding code in a Python 3 loop, however, results in an exception bubbling up the call stack and your application terminating. The Python 3 way is as follows.

Python 3.1.2 (r312:79360M, Mar 24 2010, 01:33:18) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> f = open('outfile', 'w')
>>> print('The Output', file=f)
>>>

It will take some time to get used to treating print as a function rather than a statement, but it's worth it. This now allows print to be passed around as a first class object, on par with any user-defined wrappers that would have been used previously.

This change is documented in PEP3105, which is available at http://www.python.org/dev/peps/pep-3105/.

Python's syntax for catching exceptions has been changed as well. Previously, programmers would write code similar to the following.

Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> try:
...     1/0
... except ZeroDivisionError, e:
...     print e
... 
integer division or modulo by zero

This is perfectly valid syntax; however, it often leads to bugs that are not always easy to discover during development. Consider the following code:

Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> try:
...     1/0
... except ZeroDivisionError, OSError:
...     print "Got an Error"
... 
Got an Error
>>> 

What's wrong with this? Exactly. The developer intends to catch either ZeroDivisionError or OSError. However, that's not how this is treated. Here, we actually assign the value of the caught ZeroDivisionError object to OSError! To eliminate that problem (and awkward syntax), the as keyword is now required in this situation.

Python 3.1.2 (r312:79360M, Mar 24 2010, 01:33:18) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> try:
...     1/0
... except (ZeroDivisionError, OSError) as e:
...     print(e)
... 
int division or modulo by zero
>>>

Attempting to use the syntax in the Python 2 example results in a SyntaxError exception. This ensures there is no ambiguity following the except statement.

Exception changes were proposed in PEP3110. These updates actually made it into the Python 2 series as well. More information is available at http://www.python.org/dev/peps/pep-3110/.

Metaclasses are a bit of an advanced topic; however, their syntax is worthy of mention. A metaclass is essentially a class that is responsible for building a class. Try not to think about that too hard just yet!

Previous versions of Python enforced a series of rules that would be used to determine what a class' metaclass would be. Programmers could specify one explicitly by inserting an attribute named __metaclass__ into the class definition. It was also possible to do this at the module level, which would cause all defined classes in that module to default to newstyle.

In Python 3, all classes are new style. If you have a need to specify a metaclass, you can now do so via a keyword style argument within the class definition.

Python 3.1.2 (r312:79360M, Mar 24 2010, 01:33:18) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> class UselessMetaclassStatement(metaclass=type):
...     pass
... 
>>> 

The above example, while pointless, illustrates the new syntax.

Subsets of Python tokens are now treated as reserved words and cannot be reassigned. Python 3 adds True, False, None, as, and with. The latter two were reserved as of 2.6 with a warning on None reassignment.

As should be expected, a number of modules in the standard library were updated, added, or removed. Many of them were changed to support proper PEP8-compliant naming conventions. For example, Queue becomes queue and ConfigParser becomes configparser. The list of changes is exhaustive. For a detailed look, see http://www.python.org/dev/peps/pep-3108/, which describes all of the updates.

Python's list comprehensions are a powerful feature. They've been changed in Python 3 and generally cleaned up a bit. There are two major changes that you should remember. First, loop control variables are no longer leaked.

For example, the following is valid in Python 2:

Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>> [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> i
9
>>> 

The above example would result in a NameError under Python 3 when attempting to access i outside of the list compression proper.

Also, the [i for i in 1,2,3] syntax is no longer valid. Literals like this must now be enclosed in parenthesis (making them valid tuples). So, the [i for i in (1,2,3)] should now be used.